1. Field of the Invention
The present invention relates to an image reading apparatus for photoelectrically reading a document image.
2. Description of the Related Art
In general, an image reading apparatus mounted on a digital copying machine or the like includes an imaging lens, a line sensor, and a reflecting mirror. The imaging lens and the line sensor are fixed in a body (Japanese Patent Laid-Open No. 3-113961). On the other hand, the reflecting mirror is mounted on a movable scanning unit to move in the sub-scanning direction with respect to a document. In the image reading apparatus described in Japanese Patent Laid-Open No. 3-113961, the maximum viewing angle is generally designed to be almost 20°.
FIG. 25 depicts a plan view for explaining the viewing angle in an image reading apparatus. The vertical direction of FIG. 25 corresponds to the main scanning direction of the image reading apparatus. The horizontal direction of FIG. 25 corresponds to the sub-scanning direction. The reflection angle at which a reflecting mirror 522 reflects a document image changes between end portions and the central portion in the main scanning direction. More specifically, at each end portion in the main scanning direction, the reflecting mirror 522 reflects light at a predetermined angle θ so that an imaging lens 525 can condense it. The angle θ is called a viewing angle. The viewing angle gradually decreases toward the center in the main scanning direction. Especially, the viewing angle θ is 0° at the center in the main scanning direction. That is, the viewing angle of the document image changes depending on the position where the reflecting mirror 522 reflects the light.
Recently, the size reduction of image reading apparatuses has received a great deal of attention. Japanese Patent Laid-Open No. 2004-126448 has proposed an image reading apparatus which reduces the size thereof by employing, as an imaging element, an offaxial imaging unit which forms an image via a plurality of mirrors each having an offaxial reflecting surface. An offaxial reflecting surface is a reflecting surface having a curvature and a reflecting direction different from the reference-optical-axis, ray-incident direction. Not only such a compact image reading apparatus but also a large image reading apparatus needs shading correction to correct the light amount unevenness. In general, a color image reading apparatus has, on the photoelectric conversion elements of a line sensor, three color filters to pass red (R), green (G), and blue (B) light components, respectively. An RGB line sensor receives the light components that have passed through the three color filters and photoelectrically converts them, thereby obtaining RGB luminance signals. The illuminance of a light source generally varies. In addition, the light amount around the imaging lens or the imaging mirror decreases. For these reasons, unevenness (shading) occurs in the illuminance on the imaging plane, and shading correction is necessary. In shading correction, generally, the sensor reads a white reference member immediately before reading a document. Based on the reading result, the gain and offset are adjusted for each pixel.
However, shading correction using the white reference member is effective only for light amount variations which are not related to the spectral characteristic (spectral optical characteristic) of the reading optical system, such as the illuminance unevenness of the light source or the decrease in the light amount around the imaging lens. That is, it is impossible to correct the influence of a change in the spectral characteristic caused by the difference in the viewing angle between the reflecting mirror, the imaging mirror, and the imaging lens.
FIG. 26 depicts a view illustrating a graph showing the spectral characteristics of the reflecting mirror corresponding to different viewing angles (15°, 30° and 45°). The abscissa represents the wavelength, and the ordinate represents the reflectance. As is apparent from FIG. 26, when the viewing angle becomes larger, the spectral characteristic shifts toward shorter wavelengths as a whole (the principle of wavelength shift will be described later).
The change in the spectral characteristic depends on the viewing angle when a document image becomes incident on the reflecting mirror, the imaging mirror, and the imaging lens. For this reason, the larger the viewing angle, the larger the change in the spectral characteristic. Note that the spectral characteristic of the entire reading optical system is given as the product of the spectral characteristics of all devices such as the light source, mirrors, and the image sensor included in the reading optical system. At the time of shading correction, therefore, the change in the spectral characteristic of the whole optical system depending on the viewing angle affects the spectral characteristic of the white reference member.
Especially when the light reflected by the document is light of a color (achromatic color such as white, black, or gray) having a spectral characteristic similar to that of white of the white reference member, the effect of shading correction is surely obtained. However, if the light reflected by the document is chromatic light, the main-scanning reading luminance becomes uneven even after shading correction. This is because the shading correction, which is performed based on light of the peak wavelength of white light, cannot completely correct shading of chromatic light that is different from the peak wavelength. This problem can arise both in reading a chromatic color using an RGB line sensor and in reading a chromatic color using a monochrome line sensor to be described later.
In general, when the difference in the viewing angle between the end portion and the central portion in the main scanning direction is small, the influence of the change in the spectral characteristic of the optical system depending on the viewing angle also becomes small. For example, as in the image reading apparatus described in Japanese Patent Laid-Open No. 3-113961, the difference in the viewing angle can be made smaller by prolonging the optical path from the reflecting mirror to the CCD sensor. However, since a longer optical path leads to an increase in the size of the image reading apparatus, the object to form a compact apparatus, and thus reduce the cost, cannot be achieved.
In addition to the white reference member, reference boards of red, green, and blue, or cyan, magenta, and yellow having managed densities may be provided, and a shading correction coefficient may be determined for each color. This method also enables a reduction of the unevenness in the main-scanning reading luminance of a chromatic color. In this method, however, the cost inevitably increases since the number of reference boards that need density management increases. It is also necessary to hold, in a memory, the same number of shading correction coefficients as the number of colors of the reference boards. Also required is a circuit for discriminating a color on a document and selecting a correction coefficient in accordance with the color. This makes the shading correction circuit larger and more complex.
Japanese Patent Laid-Open No. 2003-087503 has proposed an image sensor of 4-line sensor type including three color line sensors (RGB line sensors) and one monochrome line sensor. The four line sensors generally have a sensitivity difference between them due to the presence/absence of color filters. To prevent this, Japanese Patent Laid-Open No. 2003-087503 has proposed evaporating any one of RGB filters on the monochrome line sensor as well. In this case, however, the sensitivity of the monochrome line sensor decreases, and the SN (signal to noise) ratio therefore decreases in high-speed reading. Instead of evaporating a color filter on the monochrome line sensor, the monochrome reading speed may be made higher than the color reading speed. This is supposed to improve the SN ratio. However, if the reading speed of the color sensors is different from that of the monochrome line sensor, a spectral characteristic sensitivity difference is generated between the sensors depending on the presence/absence of a color filter, and the degree of occurrence of unevenness in the main-scanning reading luminance changes between them.